Vehicles typically include at least one electronic control module (ECM) that performs a wide variety of functions in assisting an operator of the vehicle in controlling the vehicle. The ECM is typically connected to a large number of sensors throughout the vehicle as well as numerous actuators. With the sensors, the ECM can determine certain information about the state of the vehicle. With the actuators, the ECM can affect the operation of the vehicle. For example, the ECM can detect heavy braking by the operator of the vehicle via a brake pedal sensor and, in response, operate the anti-lock features of the braking system.
Some of the functions performed by the ECM require or benefit from information regarding the speed of the vehicle. The speed of the vehicle can be difficult to determine precisely, however, as a speed calculation might rely on data from several different sources, any of which can be confounded during certain circumstances. For example, a wheel sensor may detect the rotational rate of the wheel. Together with information regarding the geometry of the wheel, a rate of speed for travel of the vehicle can be determined. In certain circumstances, however, the wheel may be moving independently of the vehicle's travelling speed, such as while slipping on a surface, or locked in a stopped position during a skid. In addition, during aggressive turning maneuvers, the vehicle can experiences a heavy steering and/or plowing condition, resulting in a large speed differential between wheel speeds and the vehicle's speed. Consequently, measures of the vehicle's speed which rely on wheel speeds are adversely affecting. Accordingly, estimates of the vehicle's speed that depend solely on the wheel speed can be unreliable. Similarly, accelerometers and/or transmission speed sensors can be unreliable in certain circumstances.
Although certain sensors are not helpful in determining the vehicle's speed under various circumstances, they can be used effectively during others. Accordingly, it is important to determine under what conditions the information from a specific sensor should be relied on, and when it should not. Additionally, the ECM always estimates a vehicle's speed, even when a sensor is unreliable. Accordingly, basing the estimation on a number of sensors is desirable.